RFC 7371

Updates to the IPv6 Multicast Addressing Architecture

Internet Engineering Task Force (IETF) M. Boucadair
Request for Comments: 7371 France Telecom
Updates: 3306, 3956, 4291 S. Venaas
Category: Standards Track Cisco
ISSN: 2070-1721 September 2014 Updates to the IPv6 Multicast Addressing Architecture
Abstract
This document updates the IPv6 multicast addressing architecture by
redefining the reserved bits as generic flag bits. The document also
provides some clarifications related to the use of these flag bits.
This document updates RFCs 3956, 3306, and 4291.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7371.

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Table of Contents
1. Introduction ....................................................31.1. Requirements Language ......................................32. Addressing Architecture Update ..................................33. Flag Bits: New Processing Rules .................................44. RFC Updates .....................................................44.1. Updates to RFC 3306 ........................................44.1.1. Update #1 ...........................................44.1.2. Update #2 ...........................................64.2. Updates to RFC 3956 ........................................64.2.1. Update #1 ...........................................64.2.2. Update #2 ...........................................74.2.3. Update #3 ...........................................84.2.4. Update #4 ...........................................95. Security Considerations .........................................96. Acknowledgements ................................................97. References ......................................................97.1. Normative References .......................................97.2. Informative References ....................................10

1. Introduction
This document updates the IPv6 addressing architecture [RFC4291] by
redefining reserved bits as generic flag bits (Section 2). The
document also provides some clarifications related to the use of
these flag bits (Section 3).
This document updates [RFC3956], [RFC3306], and [RFC4291]. These
updates are logical consequences of the new processing rules in
Section 3.
Textual representation of IPv6 addresses included in the RFC updates
follows the recommendation in [RFC5952].
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. Addressing Architecture Update
Bits 17-20 of a multicast address, where bit 1 is the most
significant bit, are defined in [RFC3956] and [RFC3306] as reserved
bits. This document defines these bits as generic flag bits so that
they apply to any multicast address. These bits are referred to as
"ff2" (flag field 2), while the "flgs" bits in [RFC4291] [RFC3956]
are renamed to "ff1" (flag field 1).
Within this document, flag bits denote both ff1 and ff2.
Defining the bits 17-20 as flags for all IPv6 multicast addresses
allows addresses to be treated in a more uniform and generic way, and
allows for these bits to be defined in the future for different
purposes, irrespective of the specific type of multicast address.
For the record, this design choice was initially triggered by the
specification in [ADDR-FORMAT], which proposed associating a meaning
with one of the reserved bits. Moreover, [ADDR-FORMAT] also
considered the use of the last remaining flag in ff1, but that
approach was abandoned because it is not clear at this stage whether
there are other usage scenarios of the flag.
Section 4 specifies the updated structure of the addressing
architecture.
Further specification documents may define a meaning for these
flag bits.

3. Flag Bits: New Processing Rules
Some implementations and specification documents do not treat the
flag bits as separate bits but tend to use their combined value as a
4-bit integer. This practice is a hurdle for assigning a meaning to
the remaining flag bits. Below are listed some examples for
illustration purposes:
o The reading of [RFC3306] may lead one to conclude that ff3x::/32
is the only allowed Source-Specific Multicast (SSM) IPv6 prefix
block.
o [RFC3956] states that only ff70::/12 applies to Embedded-RP.
Particularly, implementations should not treat the fff0::/12 range
as Embedded-RP.
To avoid such confusion and to unambiguously associate a meaning with
the remaining flags, the following requirement is made:
Implementations MUST treat flag bits as separate bits.
4. RFC Updates
4.1. Updates to RFC 3306
4.1.1. Update #1
This document changes Section 4 of [RFC3306] as follows:
OLD:
| 8 | 4 | 4 | 8 | 8 | 64 | 32 |
+--------+----+----+--------+--------+----------------+----------+
|11111111|flgs|scop|reserved| plen | network prefix | group ID |
+--------+----+----+--------+--------+----------------+----------+
+-+-+-+-+
flgs is a set of 4 flags: |0|0|P|T|
+-+-+-+-+
o P = 0 indicates a multicast address that is not assigned
based on the network prefix. This indicates a multicast
address as defined in [ADDRARCH].
o P = 1 indicates a multicast address that is assigned based
on the network prefix.

o If P = 1, T MUST be set to 1, otherwise the setting of the T
bit is defined in Section 2.7 of [ADDRARCH].
The reserved field MUST be zero.
Note: [ADDRARCH] is a reference listed in [RFC3306]. [ADDRARCH]
has been since obsoleted by [RFC4291].
NEW:
| 8 | 4 | 4 | 4 | 4 | 8 | 64 | 32 |
+--------+----+----+----+----+--------+----------------+----------+
|11111111|ff1 |scop|ff2 |rsvd| plen | network prefix | group ID |
+--------+----+----+----+----+--------+----------------+----------+
+-+-+-+-+
ff1 (flag field 1) is a set of 4 flags: |X|Y|P|T|
+-+-+-+-+
X and Y may each be set to 0 or 1. Note that X is for future
assignment, while a meaning is associated with Y in RFC 3956.
o P = 0 indicates a multicast address that is not assigned
based on the network prefix. This indicates a multicast
address as defined in [RFC4291].
o P = 1 indicates a multicast address that is assigned based
on the network prefix.
o If P = 1, T MUST be set to 1; otherwise, the setting of the
T bit is defined in Section 2.7 of [RFC4291].
+-+-+-+-+
ff2 (flag field 2) is a set of 4 flags: |r|r|r|r|
+-+-+-+-+
where "rrrr" are for future assignment as additional flag bits.
r bits MUST each be sent as zero and MUST be ignored on receipt.
Flag bits denote both ff1 and ff2.

4.1.2. Update #2
This document changes Section 6 of [RFC3306] as follows:
OLD:
These settings create an SSM range of FF3x::/32 (where 'x' is any
valid scope value). The source address field in the IPv6 header
identifies the owner of the multicast address.
NEW:
If the flag bits in ff1 are set to 0011, these settings create an
SSM range of ff3x::/32 (where 'x' is any valid scope value). The
source address field in the IPv6 header identifies the owner of
the multicast address. ff3x::/32 is not the only allowed SSM
prefix range. For example, if the most significant flag bit in
ff1 is set, then we would get the SSM range ffbx::/32.
4.2. Updates to RFC 3956
4.2.1. Update #1
This document changes Section 2 of [RFC3956] as follows:
OLD:
As described in [RFC3306], the multicast address format is
as follows:
| 8 | 4 | 4 | 8 | 8 | 64 | 32 |
+--------+----+----+--------+----+----------------+----------+
|11111111|flgs|scop|reserved|plen| network prefix | group ID |
+--------+----+----+--------+----+----------------+----------+
Where flgs are "0011". (The first two bits are as yet undefined,
sent as zero and ignored on receipt.)

NEW:
The multicast address format is as follows:
| 8 | 4 | 4 | 4 | 4 | 8 | 64 | 32 |
+--------+----+----+----+----+----+----------------+----------+
|11111111|ff1 |scop|ff2 |rsvd|plen| network prefix | group ID |
+--------+----+----+----+----+----+----------------+----------+
+-+-+-+-+
ff1 (flag field 1) is a set of four flags: |X|R|P|T|
+-+-+-+-+
where X is for future assignment as an additional flag bit.
X may be set to 0 or 1.
+-+-+-+-+
ff2 (flag field 2) is a set of 4 flags: |r|r|r|r|
+-+-+-+-+
where "rrrr" are for future assignment as additional flag bits.
r bits MUST each be sent as zero and MUST be ignored
on receipt.
Flag bits denote both ff1 and ff2.
4.2.2. Update #2
This document changes Section 3 of [RFC3956] as follows:
OLD:
| 8 | 4 | 4 | 4 | 4 | 8 | 64 | 32 |
+--------+----+----+----+----+----+----------------+----------+
|11111111|flgs|scop|rsvd|RIID|plen| network prefix | group ID |
+--------+----+----+----+----+----+----------------+----------+
+-+-+-+-+
flgs is a set of four flags: |0|R|P|T|
+-+-+-+-+
When the highest-order bit is 0, R = 1 indicates a multicast address
that embeds the address on the RP. Then P MUST be set to 1, and
consequently T MUST be set to 1, as specified in [RFC3306]. In
effect, this implies the prefix FF70::/12. In this case, the last 4
bits of the previously reserved field are interpreted as embedding
the RP interface ID, as specified in this memo.

The behavior is unspecified if P or T is not set to 1, as then the
prefix would not be FF70::/12. Likewise, the encoding and the
protocol mode used when the two high-order bits in "flgs" are set to
11 ("FFF0::/12") is intentionally unspecified until such time that
the highest-order bit is defined. Without further IETF
specification, implementations SHOULD NOT treat the FFF0::/12 range
as Embedded-RP.
NEW:
| 8 | 4 | 4 | 4 | 4 | 8 | 64 | 32 |
+--------+----+----+----+----+----+----------------+----------+
|11111111|ff1 |scop|ff2 |RIID|plen| network prefix | group ID |
+--------+----+----+----+----+----+----------------+----------+
+-+-+-+-+
ff1 is a set of four flags: |X|R|P|T|
+-+-+-+-+
where X is for future assignment as an additional flag bit.
X may be set to 0 or 1.
R = 1 indicates a multicast address that embeds the address of the
RP. Then, P MUST be set to 1, and consequently T MUST be set
to 1, according to [RFC3306], as this is a special case of
unicast-prefix-based addresses. This implies that, for instance,
prefixes ff70::/12 and fff0::/12 are embedded RP prefixes. When
the R-bit is set, the last 4 bits of the field that were reserved
in [RFC3306] are interpreted as embedding the RP interface ID, as
specified in this memo.
4.2.3. Update #3
This document changes Section 4 of [RFC3956] as follows:
OLD:
o It MUST be a multicast address with "flgs" set to 0111, that is, to
be of the prefix FF70::/12,
NEW:
o It MUST be a multicast address with the R-bit set to 1.
o It MUST have the P-bit and T-bit both set to 1 when using the
embedding in this document as it is a prefix-based address.

4.2.4. Update #4
This document changes Section 7.1 of [RFC3956] as follows:
OLD:
To avoid loops and inconsistencies, for addresses in the range
FF70::/12, the Embedded-RP mapping MUST be considered the longest
possible match and higher priority than any other mechanism.
NEW:
To avoid loops and inconsistencies, for addresses with the R-bit
set to 1, the Embedded-RP mapping MUST be considered the longest
possible match and higher priority than any other mechanism.
5. Security Considerations
The same security considerations as those discussed in [RFC3956],
[RFC3306], and [RFC4291] are to be taken into account.
6. Acknowledgements
Special thanks to Brian Haberman for the discussions prior to the
publication of this document.
Many thanks to Jouni Korhonen, Tatuya Jinmei, Charlie Kaufman, and
Ben Campbell for their review.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3306] Haberman, B. and D. Thaler, "Unicast-Prefix-based IPv6
Multicast Addresses", RFC 3306, August 2002.
[RFC3956] Savola, P. and B. Haberman, "Embedding the Rendezvous
Point (RP) Address in an IPv6 Multicast Address",
RFC 3956, November 2004.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[RFC5952] Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
Address Text Representation", RFC 5952, August 2010.